Interconnect structures in integrated circuits induce a delay in the propagation of the information between semiconductor devices such as transistors. To reduce this delay, the interconnect structures should posses the lowest capacitance possible. One approach to form interconnect structures with the lowest possible capacitance is to introduce air (or vacuum) gaps into the interconnect dielectric material of the interconnect structure; by replacing a portion of the dielectric material with an air gap, the capacitance can be reduced dramatically.
Among several integration schemes proposed for air gap integration, a dielectric etch back scheme is the most commonly used. See, for example, Arnal et al, 2001 Proc. IEEE International Interconnect Technology Conference, pages 298-300. In this scheme, the gap is etched in a dielectric cap between two metal lines, and eventually transferred into the dielectric material. After transferring the gap into the dielectric material, the dielectric material is isotropically removed between the two metal lines. A dielectric deposition process is then performed that pinches off the gap such that the next interconnect level can be fabricated.
There are several problems with the aforementioned prior art air gap fabrication scheme. First, the above mentioned prior art air gap fabrication scheme requires a very accurate lithographic step to define trenches between the metal lines. Furthermore, the above mentioned prior art scheme requires an etch step to remove the dielectric in between the metal lines. If the lithography step is not perfect (misaligned, or too large an opening may result) and, if the etch step leads to a width increase, the metal lines can be damaged by this etch step. The isotropic etch process may also damage the sidewalls of the metal line or the interface between the metal lines and an overlying cap layer. Both issues may lead to reliability issues.
Finally, the need for very high resolution lithography and a critical etch step leads to a huge increase in the cost of the air gap module. Thus, there is a need for a simplified integration scheme that leads to cheap and reliable air gap-containing interconnect structures.